STEADY FLOW OF A VISCOUS ICE STREAM ACROSS A NO-SLIP FREE-SLIP TRANSITION AT THE BED

An exact, analytic description of the steady, downhill flow of a visco us ice stream across a no-slip to free-slip transition in basal bounda ry condition suggests ways in which such transitions might bc detected by observations of ice-stream surface properties. We find that the be st expression of this transition is in the free-surface profile, which dips over the point of transition and becomes horizontal far downstre am. The streamwise velocity at the surface shows a gradual change acro ss the transition, and this is in disagreement with previous studies w hich suggest a marked change. Basal shear stress and basal pressure ex hibit singularities at the point of transition. As concluded in previo us study, the former singularity is a likely point of strong basal ero sion. The latter singularity is problematical, however, because it vio lates the assumptions which make the exact solution possible. It is no t clear how this problem can be overcome without appealing to thermody namics, non-Newtonian material properties, cavitation or wake separati on.